Bright days ahead for the Amiga world. AROS is doing well, AmigaOS4 is getting one heck of a machine in the AmigaOne X1000, and MorphOS continues its development at a brisk pace. Version 2.6 of MorphOS, currently in development, will add support for (G4, I'm assuming) PowerMacs, which, alongside support for the Mac Mini and eMac, gives MorphOS a solid base of used hardware to run on.

-PowerPC is dead for desktop use. Apple ditched it when its power management performance became unsatisfactory, and Sony remotely killed OtherOS on the PS3 because they feared some random piracy danger and actually thought that average people would actually go as far as installing linux on their PS3 in order to pirate games (well, it's Sony after all). The only remaining PowerPC desktops are legacy hardware that you'll likely be unable to replace once it wears out.

-This makes x86(_64) the only widespread desktop computer architecture, since other architectures (SPARC, ARM...) have not much presence on that market (should I say "yet" about ARM ?)

-Hence people replacing their dead PowerPC hardware will likely buy x86 hardware next, making x86 support more of a priority than legacy G4s